Dynamics of egg counts and circulating antigen levels in a recent Schistosoma mansoni focus in northern Senegal

Serum circulating anodic antigen (CAA) levels were compared with faecal egg counts in four subsequent population samples, randomly selected at 8-month intervals, in a recent Schistosoma mansoni focus in northern Senegal. In a ll four samples, antigen levels showed the same age-intensity profiles as e gg counts, with a strong decline in adults. Also across population samples, a consistent relationship was found between egg counts and antigen levels. Assuming the level of CAA to be a direct reflection of worm burden, these findings support the idea that the observed egg count patterns and levels i ndeed reflect dynamics of worm burdens, and not of egg excretion or worm fe cundity. Remarkably similar levels of both egg counts and CAA were observed in the first and last sample, collected in the same season (August-Septemh er), hut 2 years apart. This suggests that a steady state of S. mansoni inf ection had already been reached shortly after the onset of the epidemic in this focus (3 years). Significantly lower infection levels were found in th e intermediate population samples collected in January and April. The diffe rences in infection levels across the four population samples may be becaus e of seasonal transmission patterns. They would indicate a substantial turn over of worm populations, with an estimated average life span of only 7 mon ths, probably less, in this recently emerged, intense S. mansoni focus.